How to bulk rename objects within a target collection?
Iterate over collection objects.

This example is a reasonably simple introduction into scripting blender's python API.

import bpy

wcol = bpy.data.collections.get("Water")
if wcol:
    for i, o in enumerate(wcol.objects):
        o.name = "Water%d" % i


To ensure they keep their previous name order, and to have "Water1" first rather than "Water0"

import bpy

wcol = bpy.data.collections.get("Water")
if wcol:
    for i, o in enumerate(sorted(wcol.objects[:], key=lambda o: o.name)):
        o.name = "Water%d" % (i + 1) # to start from 1